Widthdom · Widthdom · Jun 6, 2026 · Jun 6, 2026 · Jun 6, 2026 · Jun 6, 2026
diff --git a/changelog.d/unreleased/3055.fixed.md b/changelog.d/unreleased/3055.fixed.md
@@ -0,0 +1,18 @@
+---
+category: fixed
+issues:
+  - 3055
+affected:
+  - src/CodeIndex/Mcp/McpJsonNode.cs
+  - src/CodeIndex/Mcp/McpErrorEnvelope.cs
+  - src/CodeIndex/Mcp/McpServer.cs
+  - tests/CodeIndex.Tests/McpServerTests.cs
+---
+
+## English
+
+- **MCP JSON node cloning no longer serializes and reparses valid nodes (#3055)** — MCP response ids, session capabilities, progress tokens, and structured error extra data now use `JsonNode.DeepClone()` through a shared helper instead of `ToJsonString()` plus `JsonNode.Parse(...)`.
+
+## 日本語
+
+- **MCP の JSON node clone が有効な node を文字列化して再 parse しなくなりました (#3055)** — MCP response id、session capabilities、progress token、structured error の追加データは、`ToJsonString()` と `JsonNode.Parse(...)` の組み合わせではなく、共有 helper 経由の `JsonNode.DeepClone()` を使うようになりました。
diff --git a/src/CodeIndex/Mcp/McpErrorEnvelope.cs b/src/CodeIndex/Mcp/McpErrorEnvelope.cs
@@ -76,7 +76,7 @@ public static JsonObject BuildData(string category, string suggestion, bool retr
                 // canonical キーは extra でも上書きさせない。
                 if (kvp.Key is "category" or "suggestion" or "retry_safe")
                     continue;
-                data[kvp.Key] = kvp.Value is null ? null : JsonNode.Parse(kvp.Value.ToJsonString());
+                data[kvp.Key] = McpJsonNode.Clone(kvp.Value);
             }
         }
         return data;

diff --git a/src/CodeIndex/Mcp/McpJsonNode.cs b/src/CodeIndex/Mcp/McpJsonNode.cs
@@ -0,0 +1,9 @@
+using System.Text.Json.Nodes;
+
+namespace CodeIndex.Mcp;
+
+internal static class McpJsonNode
+{
+    public static JsonNode? Clone(JsonNode? node)
+        => node?.DeepClone();
+}
diff --git a/src/CodeIndex/Mcp/McpServer.cs b/src/CodeIndex/Mcp/McpServer.cs
@@ -1088,7 +1088,7 @@ private bool TryCloneClientResponsePayload(JsonNode? payload, out JsonNode? clon
         if (!TryMeasureJsonUtf8BytesWithinLimit(payload, _jsonOptions, MaxClientResponseJsonBytes, out bytesWritten))
             return false;
 
-        clone = payload.DeepClone();
+        clone = McpJsonNode.Clone(payload);
         return true;
     }
 
@@ -1212,7 +1212,7 @@ private static void ExtractResponseId(JsonNode request, out bool hasId, out Json
         if (request is JsonObject obj)
         {
             if (TryGetRequestId(obj, out hasId, out var requestId))
-                id = requestId is null ? null : JsonNode.Parse(requestId.ToJsonString());
+                id = McpJsonNode.Clone(requestId);
             else
                 id = null;
             return;
@@ -1916,23 +1916,51 @@ private void CaptureClientSession(JsonNode? initializeParams)
     private void CaptureClientCapabilities(JsonNode capabilities)
     {
         CaptureClientCapabilityFlags(capabilities);
-        var json = capabilities.ToJsonString();
-        var serializedBytes = Encoding.UTF8.GetByteCount(json);
-        _clientCapabilitiesSerializedBytes = serializedBytes;
-        if (serializedBytes > MaxClientCapabilitiesJsonBytes)
+        if (!TryMeasureJsonUtf8BytesWithinLimit(capabilities, _jsonOptions, MaxClientCapabilitiesJsonBytes, out var serializedBytes))
         {
+            _clientCapabilitiesSerializedBytes = serializedBytes;
             TruncateClientCapabilities("byte_limit");
             return;
         }
 
-        try
+        _clientCapabilitiesSerializedBytes = serializedBytes;
+        if (!IsJsonNodeDepthWithinLimit(capabilities, MaxClientCapabilitiesDepth))
         {
-            _clientCapabilities = JsonNode.Parse(json, documentOptions: new JsonDocumentOptions { MaxDepth = MaxClientCapabilitiesDepth });
+            TruncateClientCapabilities("depth_limit");
+            return;
         }
-        catch (JsonException)
+
+        _clientCapabilities = McpJsonNode.Clone(capabilities);
+    }
+
+    private static bool IsJsonNodeDepthWithinLimit(JsonNode node, int maxDepth)
+        => IsJsonNodeDepthWithinLimit(node, depth: 0, maxDepth);
+
+    private static bool IsJsonNodeDepthWithinLimit(JsonNode? node, int depth, int maxDepth)
+    {
+        if (node is null)
+            return true;
+        if (depth > maxDepth)
+            return false;
+
+        if (node is JsonObject obj)
         {
-            TruncateClientCapabilities("depth_limit");
+            foreach (var kvp in obj)
+            {
+                if (!IsJsonNodeDepthWithinLimit(kvp.Value, depth + 1, maxDepth))
+                    return false;
+            }
+        }
+        else if (node is JsonArray array)
+        {
+            foreach (var item in array)
+            {
+                if (!IsJsonNodeDepthWithinLimit(item, depth + 1, maxDepth))
+                    return false;
+            }
         }
+
+        return true;
     }
 
     private void TruncateClientCapabilities(string reason)
@@ -1973,7 +2001,7 @@ private void ResetClientRoots()
         _clientRootsTruncated = false;
     }
 
-    internal JsonNode? ClientCapabilitiesForTests => _clientCapabilities is null ? null : JsonNode.Parse(_clientCapabilities.ToJsonString());
+    internal JsonNode? ClientCapabilitiesForTests => McpJsonNode.Clone(_clientCapabilities);
 
     internal string[] ClientRootsForTests => _clientRoots
         .Select(root => root?.GetValue<string>())
@@ -2426,7 +2454,7 @@ private static JsonObject CreateUnsupportedProtocolError(JsonNode? id, BoundedMc
         {
             ["jsonrpc"] = "2.0",
             ["error"] = error,
-            ["id"] = id is null ? JsonNode.Parse("null") : JsonNode.Parse(id.ToJsonString())
+            ["id"] = McpJsonNode.Clone(id)
         };
         return response;
     }
@@ -2503,7 +2531,7 @@ internal static JsonObject CreateRateLimitedErrorResponse(JsonNode? id, string t
         {
             ["jsonrpc"] = "2.0",
             ["error"] = error,
-            ["id"] = id is null ? JsonNode.Parse("null") : JsonNode.Parse(id.ToJsonString())
+            ["id"] = McpJsonNode.Clone(id)
         };
         return response;
     }
@@ -2790,7 +2818,7 @@ private void EmitToolInvocationTelemetry(string toolName, JsonNode? args, JsonNo
             return null;
 
         return TryMeasureJsonUtf8BytesWithinLimit(token, _jsonOptions, McpBoundedText.MaxProgressTokenJsonBytes, out _)
-            ? token.DeepClone()
+            ? McpJsonNode.Clone(token)
             : null;
     }
 
@@ -2854,7 +2882,7 @@ private void EmitProgressNotification(JsonNode? progressToken, long progress, lo
 
         var parameters = new JsonObject
         {
-            ["progressToken"] = progressToken.DeepClone(),
+            ["progressToken"] = McpJsonNode.Clone(progressToken),
             ["progress"] = progress,
         };
         if (total.HasValue)
@@ -3578,7 +3606,7 @@ private static JsonObject CreateSuccessResponse(bool hasId, JsonNode? id, JsonNo
             ["result"] = result
         };
         if (hasId)
-            response["id"] = id is null ? JsonNode.Parse("null") : JsonNode.Parse(id.ToJsonString());
+            response["id"] = McpJsonNode.Clone(id);
         return response;
     }
 
@@ -3676,7 +3704,7 @@ private static JsonObject CreateErrorResponse(bool hasId, JsonNode? id, int code
             }
         };
         if (hasId)
-            response["id"] = id is null ? JsonNode.Parse("null") : JsonNode.Parse(id.ToJsonString());
+            response["id"] = McpJsonNode.Clone(id);
         return response;
     }
 

diff --git a/tests/CodeIndex.Tests/McpServerTests.cs b/tests/CodeIndex.Tests/McpServerTests.cs
@@ -868,6 +868,59 @@ public void Initialize_CapturesClientCapabilitiesAndRootsForSessionStatus()
         Assert.Equal("info", session["log_level"]!.GetValue<string>());
     }
 
+    [Fact]
+    public void Initialize_CapturesClientCapabilitiesAsDetachedClone_Issue3055()
+    {
+        var capabilities = new JsonObject
+        {
+            ["experimental"] = new JsonObject
+            {
+                ["progress"] = true,
+            },
+        };
+        var request = new JsonObject
+        {
+            ["jsonrpc"] = "2.0",
+            ["id"] = 1,
+            ["method"] = "initialize",
+            ["params"] = new JsonObject
+            {
+                ["capabilities"] = capabilities,
+            },
+        };
+
+        _server.HandleMessage(request);
+        capabilities["experimental"]!["progress"] = false;
+        var copy = _server.ClientCapabilitiesForTests!;
+        copy["experimental"]!["progress"] = false;
+
+        Assert.True(_server.ClientCapabilitiesForTests!["experimental"]!["progress"]!.GetValue<bool>());
+    }
+
+    [Fact]
+    public void ErrorEnvelope_ClonesExtraDataWithoutSerializeParseRoundTrip_Issue3055()
+    {
+        var details = new JsonObject
+        {
+            ["ok"] = true,
+        };
+        var extra = new JsonObject
+        {
+            ["details"] = details,
+            ["category"] = "shadow",
+        };
+
+        var data = McpErrorEnvelope.BuildData(
+            "custom_category",
+            "custom suggestion",
+            retrySafe: true,
+            extra);
+        details["ok"] = false;
+
+        Assert.Equal("custom_category", data["category"]!.GetValue<string>());
+        Assert.True(data["details"]!["ok"]!.GetValue<bool>());
+    }
+
     [Fact]
     public void Initialize_CapsClientRootsForSessionStatus_Issue3076()
     {